Method of separating a low density fly ash fraction from an overall group of fly ash

ABSTRACT

A method of separating a low density fly ash fraction from an overall group of fly ash entrained in a gas stream including the steps of passing the gas through a duct having a first area; passing the gas stream from the duct through an expansion nozzle; passing the gas stream through the expansion nozzle and into a hollow body having a second area, whereby the second area is larger than the first area; decelerating the gas stream as it enters the expansion nozzle to separate out low density ash particles; and collecting the low density ash particles from a hopper positioned adjacent the expansion nozzle.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to a method and apparatus for separating the lowerdensity fly ash particles from the overall mixture of various densityparticles contained in raw coal ash. More particularly, the particles offly ash separated and collected by this method include generally solidparticles with pervasive internal and external porosity that are frothyin appearance, thick walled hollow particles with a specific gravity inthe range of from 1.0 to 2.0 and a minor amount of thin walled hollowparticles with a specific gravity of less than 1.0 known as cenospheres.Specifically, the method involves changing the momentum of the exhaustgas; either by drastically changing the diameter of the feed stream todrastically change the velocity of the gas, or by de-energizing at leastthe first static electric charging field of an electrostaticprecipitator (ESP) through which the exhaust gases pass on their way tothe stack of the power plant whereby these hollow, porous and other lowdensity particles fall out and are collected in a collection hopper.

2. Background Information

Various types of filler materials have been used for decades to improvethe properties and lower costs of various industrial and consumerproducts including concrete blocks; plastic composites such as showerstalls, automobile body panels, sinks and countertops; roofingmaterials; tires and other rubber products; caulking compounds; paper;and a multitude of other applications. For example, in plasticcomposites these fillers are commonly utilized to enhance theirstructural and mechanical properties, to improve the composite's fireresistance, to thicken or stiffen the pre-formed mix prior to moldingand to reduce costs.

These fillers include natural or mineral fillers such as clay, talc andcalcium carbonate, and synthetic fillers, such as glass beads, groundpolymers and ceramics. Both mineral and synthetic fillers have proven tobe useful fillers in materials ranging from ceramics to a variety ofplastics including thermosetting plastics such as polyesters, epoxiesand phenolics and thermoplastics such as polyethylene, polypropylene,acrylic lattices, as well as many other resin systems.

Generally, the selection of a filler for a specific application is basedupon its physical characteristics (e.g. color, density, shape,thixotropic effect, reactivity, particle size distribution and handlingfeatures) and the mechanical properties of the filler (e.g. hardness andstrength) and the resulting properties of the filled system. One of theimportant properties of a filler is its specific gravity.

When specific gravity is examined, two classes of fillers becomeapparent, namely high density mineral fillers, and low densitymaterials, typically processed minerals or synthetic materials.

Most of the commonly used and economically priced fillers are highdensity minerals, usually having a specific gravity of 2.6 and higher.They are generally mined, are plentiful and include calcium carbonate(limestone), clay, silica and talc.

However, when the overall weight of the filled system is a concern,lower density fillers are much more desirable. As a result, the demandfor economically priced low density fillers is increasing. Low densityfillers are generally not plentiful in nature and therefore are man-madeor derived by processing natural minerals. Low density fillers aregenerally expensive and include glass or plastic microspheres, hollowglass beads, expanded ceramic spheres and cenospheres.

Raw fly ash, a product created when coal is combusted in a generationfacility is plentiful, and is being generated from coal fired powerplants. More particularly, there are currently hundreds of coal firedpower plants in the U.S. alone. These plants burn well in excess of onebillion tons of coal per year, and as such, coal combustion by-productsincluding fly ash have become one of the nation's most abundantresources. Growth is further expected as nuclear power loses preferenceto more standard power sources such as coal. As a result, the need tosafely dispose of fly ash, and the need to develop an economical use forfly ash is ever increasing. Fly ash is currently used as a fillermaterial in many applications but has never achieved the status of amajor filler. Some of the reasons why fly ash have been unable tocapture a large portion of the filler market are its color, itsdifficulty to handle due to dustiness, its wide particle sizedistribution, its hardness, and the inconsistency of its composition andproperties. Most sources of fly ash have a specific gravity in the rangeof 2.1-2.3, which is less; that than the common mineral filler, but thisapparent advantage is not enough to offset the disadvantages.

Fly ash with a low specific gravity, whether hollow or solid has beenusually separated from raw ash using water as the separation medium.Those particles having a specific gravity less than 1.0 will float andthe remaining portion of the ash sinks to the bottom of the separationpond. The cenospheres must be collected from the ponds, usually by askimming process, cleaned of other floating materials, dried, and oftenfurther processed. Cenospheres, when they are present in raw fly ash,typically represent one percent or less of the weight of the ash.However,. cenospheres have a specific gravity of less that 1.0, and arevery valuable as a low density filler, selling at a price significantlyhigher than other types of ash. Similarly, all low density ash, whencleaned and processed, is extremely valuable as a filler material.

Functionally, fly ash is the finely divided ash material carried in thestack gases from the furnace of power plants which consume powdered andpulverized coal, and is collected before it leaves the stack usually inan electrostatic, precipitator or other type of collector. The problemassociated with disposal of fly ash is large because the tonnageproduced by some utility companies is quite high. Numerous attempts havebeen made to utilize the material, and the suggestion that lightweightaggregate materials might be prepared from fly ash is the result of suchan attempt.

The present invention provides a method for economically separating alow density fraction of ash from the entire particulate range of fly ashcreated as a by-product from the combustion of coal. The resultant lowdensity ash has a specific gravity of 1.6-1.99. More particularly, theresultant ash is at least ten percent lower in density than raw fly ashand produces very little dust when handled. Although not as light as thethin walled cenospheres, the ash created from the present invention issignificantly more durable, and is primarily composed of a frothy,relatively solid particles with both internal and external porosity andlarger thick walled hollow particles. In addition to its increaseddurability over cenospheres, the material created by the presentinvention is plentiful, comprising more than thirty percent of the totalraw fly ash produced, and is less expensive to produce and process thancenospheres. The product created by the present invention thus hasadvantages when compared to both raw fly ash and cenospheres, making ita valuable low density filler produced in an economical manner.

The need thus exists for an ash that which is relatively inexpensive toproduce, which may be separated from the existing raw fly ash withlimited cost, and which provides for a low specific gravity for use as afiller material. The need also exists for an apparatus and system forseparating the above described ash for collection and use as a fillermaterial.

SUMMARY OF THE INVENTION

Objectives of the invention include providing an improved device, systemand method of separating and collecting the lower density fly ashparticles including semi-solid frothy particles with internal andexternal porosity, thick walled hollow particles and a smaller amount ofthin walled hollow particles (cenospheres) from the other higher densitysolid particles contained in raw fly ash.

A further objective is to provide a low density ash particle separationand collection method and device that effectively separates theparticles by size and density, without the use of water and subsequentde-watering and drying processes.

A further objective is to provide such a low density ash particleseparation and collection method and device that lessens the amount ofash disposed in land fills.

A further objective is to provide such a low density ash particle whichhas both internal and external porosity.

A still further objective is to provide a mixture of low density ashparticles having a size in the range of from 30 microns to 400 microns.

A further objective is to provide such a low density ash particleseparation and collection method and device that produces an inexpensivesource of low density mineral filler.

A further objective is to provide such a low density ash particleseparation and collection method and device that produces largequantities of low density ash particles.

A further objective is to provide such a low density ash particleseparation and collection method and device that produces a low densityfiller that is useful in plastics, ceramics, concrete and othermaterials.

A further objective is to provide such a low density ash particleseparation and collection method and device that is inexpensive tooperate and uses existing equipment such as electrostatic precipitators.

A further objective is to provide such a low density ash particleseparation and collection method and device that incorporates one ormore or all of the above objectives and advantages.

A further objective is to provide such a low density ash particleseparation and collection method and device which accomplishes itsobjective in a drag environment.

A further objective is to provide such a low density ash particleseparation collection method and device which substantially decreasesthe momentum of the gas stream in order to remove low density fly ashfrom the gas stream.

These and other objectives and advantages of the invention are obtainedby the invention which includes a method of separating lower density ashfrom the exhaust gas stream which contain the entrained fly ash, andthen collecting the low density ash that drops out below thede-energized fields of the electrostatic precipitator. The inventionfurther includes an electrostatic precipitator or similar device that isdesigned in such a manner to cause the gas or air stream to rapidlydecelerate causing the entrained particles to slow down and fall outsuspension. The low density fly ash particles having less momentum(relative to their size and therefore subject to greater decelerationdue to drag forces) tend to fall out sooner (i.e. in the inlet portion,first hoppers, of the precipitator or similar device). The denser ashparticles, with greater momentum and affected to a lesser extent by dragforces, are carried further into the precipitator. The precipitator orother device is, in effect, a hollow body with a larger cross sectionalarea than the inlet and outlet ducts. The electrostatic precipitatorfurther includes a plurality of energizable fields associated with oneor more collecting plates longitudinally positioned next to one anotherwithin the hollow body where the plurality of energizable fieldsincludes a first energizable field nearest the inlet. The electrostaticprecipitator further includes a de-energizing mechanism forde-energizing the first energizable field(s) resulting in the separationof the lower density ash from the overall ash stream. The ESP isequipped with collection hoppers below the plurality of energizablefields. The ash falls out of the precipitator body, due to gravity, andis collected in the hoppers. Typically a hopper would span onecollecting plate having one or more electrostatic fields associatedtherewith. The ash can be removed from individual hoppers or from rowsof hoppers by modifying the existing transport system or by installing aseparate system to extract the collected low density ash.

BRIEF DESCRIPTION OF THE DRAWINGS

The file of this patent contains at least one drawing executed in color.Copies of this patent with color drawings are provided to the Patent andTrademark Office with payment of the necessary fee.

Preferred embodiment of the invention, illustrative of the best mode inwhich applicant has contemplated applying the principles, is set forthin the following description and is shown in the drawings and isparticularly and distinctly pointed out and set forth in the appendedclaims.

FIG. 1 is a sectional view of the electrostatic precipitator of thepresent invention;

FIG. 2 is a photograph of the low density ash of the present invention;

FIG. 3 is a photograph showing thick wall hollow particles havinginternal and external pores;

FIG. 4 is a photograph depicting cenospheres; and

FIG. 5 is a photograph depicting solid low density particles.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention is a device and method of separating and collecting lowdensity fly ash particles from the overall and unprocessed raw fly ashstream produced by coal burning in a coal fired, steam generating,electric power plant by setting out such low density fly ash particlesfrom the raw coal ash stream using the de-energized inlet portion of anelectrostatic precipitator as a separating device. This is specificallyaccomplished using a method and device that utilizes velocitydeceleration to separate and collect the low density fly ash from theraw coal fly ash which will be collected by energized fields of theelectrostatic precipitator.

The invention includes an electrostatic precipitator 10 as shown in oneembodiment in FIG. 1. The electrostatic precipitator 10 generallyincludes an inlet or expansion nozzle 11, an outlet or outlet nozzle 12,and an enlarged hollow body 13 therebetween which is divided intomultiple fields 14A, 14B, 14C, etc. that are energizable as describedbelow. Many different varieties and constructions of electrostaticprecipitators have been or are available on the market, all of which mayincorporate this invention as described herein. The variety ofprecipitators available is large and includes, for example, models suchas those described in the following U.S. Pat. Nos.: 1,298,409,1,381,660, 3,898,060, 4,308,036, 4,374,652, and 5,160,510, as well asany other commercially available electrostatic precipitator.

In more detail, the electrostatic precipitator 10 is fluidly connectedwithin an exhaust duct on a coal fired power plant that is between acombustor 21, or other combustion chamber, and a stack 23 which releasesexhaust to the atmosphere 22. The exhaust duct is thus broken into aninlet duct 20, the precipitator 10, and an outlet duct 25. This istypically accomplished by cutting the exhaust duct into two pieces,namely a first section or inlet duct 20 and a second section or outletduct 25, with the electrostatic precipitator 10 attached therebetweenwhereby the inlet duct 20 separates the combustor 21 from theelectrostatic precipitator 10 while the outlet duct 25 separates theelectrostatic precipitator 10 from the stack 23 and subsequently theatmosphere 22. An exhaust gas-to-air heat exchanger is typically used topreheat the air for combustion, and can be either on the inlet or outletside of the precipitator, such as within the inlet duct 20 or the outletduct 25.

The inlet and outlet ducts 20 and 25 are generally of a similar diameterand cross sectional area, while the diameter and cross-sectional area ofthe hollow body 13 is larger so as to substantially slow down the fluidflow therein due to an overall increase in volume. More particularly,the cross sectional area of hollow body 13 is in the range of from sixto twenty-five times larger than the duct. More particularly, the largediameter of expansion nozzle 11 and hollow body 13 has an area at leastten times larger than the area associated with inlet duct 20. Basically,the flow or velocity of fluid, in the form of gas with particulatesuspended therein, must be sufficiently slowed for the electrostaticprecipitator to properly charge the particles, and this is accomplishedby a significant increase in volume in the expansion nozzle 11 to thebody 13. Thereafter, the flow is increased in the outlet nozzle 12 bydecreasing the exhaust cross section. In one embodiment, the gas flow inthe inlet duct 20 is approximately 60 ft./sec. while it falls to 2.5 to5 ft./sec. in the precipitator body 13.

The electrostatic precipitator 10 is longitudinally subdivided into thefields between expansion nozzle 11 and outlet nozzle 12. As is standardfor electrostatic precipitator, each field 14A, 14B, 14C, etc. includesone or more power supplies and each power supply is connected to one ormore independently energizable portions known as bus sections. Thenumber of fields is not important as the precipitator may only have afew or may have in excess of a dozen, while most standard precipitatorshave six or eight.

Per this longitudinal subdivision, the fields are aligned one afteranother from the expansion nozzle 11 to the outlet nozzle 12 with thefirst field 14A being adjacent or nearest the expansion nozzle 11, thesecond field 14B being next to the first field, and so on across thehollow body 13 to the outlet nozzle 12. Each field 14A, 14B, 14C, etc.is an arrangement of collecting surfaces and discharge electrodes as iswell known in the art.

In use, the fields 14A, 14B, 14C, etc. are energized with high voltageby a transformer-rectifier combination that is electrically attached toa variety of controls and other electrical components. The energizing ofthe fields as the products of coal combustion in the form of exhaustgases with suspended particles pass through electrostatic precipitator10 basically imparts a charge on the particles within the gas stream toseparate out particles. This is well known in the industry as it hasbeen in use for years, and ash separation from exhaust gas is requiredby environmental regulations. For these reasons, a large percentage ofcoal fired power plants have electrostatic precipitators on each exhaustduct after the combustor.

The electrostatic precipitator is positioned in a horizontal manner suchthat the gases must travel horizontally or approximately horizontallythrough it. This allows for particle fallout. Below or at the bottom ofeach field, or a collection of fields, is a hopper or collectionchamber, namely hopper 30A for field 14A, hopper 30B for field 14B, etc.These hoppers are separated by walls 31. Each hopper basically collectsany fall out that occurs above it as gases and particles pass throughthe precipitator and the specific field above it.

Electrostatic fields are created when the precipitator's charging wiresare energized. Particles passing these charged wires are imparted withelectrostatic charge. As a result, the charged particles are collectedand later dislodged from grounded plates and the particles fall into thehoppers while the gaseous components of the flue gas pass through andare exhausted to the atmosphere 22. Basically, only non-particulatematter passes all the way through the precipitator 10 to the outlet 12and thus is exhausted to the atmosphere 22.

In accordance with one of the features of the invention, theelectrostatic precipitator 10 is oversized. Oversized being defined ashaving excess capacity in that either lesser capacity or lesser numberof fields are needed to fully remove the particles from the exhaustgases of the coal combustion process in order to meet environmentalregulations. An oversized electrostatic precipitator 10 has fields thatmay be de-energized while still performing all of its particulateremoval objectives.

In accordance with yet another feature of the invention, the first field14A is de-energized by turning off the electric power to section 14A byoperating breaker A or other similar device. As a result, a first areais provided in the precipitator body 13 that does not impart charges onthe particles but is of a significantly increased cross sectionaldimension, thus decelerating the exhaust gases containing the particles.In this first field when de-energized, a large portion of the lowdensity ash 40 falls out into hopper 30A and accumulates as captured lowdensity ash 45 while almost all of the higher density ash particles 41,due to greater momentum and less drag, pass into the next fields wherethey are charged and fall out into the hoppers 30B, 30C, etc. ascaptured higher density ash 46. The first hopper 30A may then beevacuated using a conveying system and the contents packaged as lowdensity mineral material.

Referring specifically to the fly ash, it is, as stated, the dust-likematerial collected from the gas stream to be passed out of the stack,leaving the furnace of power plants which consume coal, and specificallypowdered and pulverized coal. Chemically, fly ash is essentially amixture of silica, aluminum oxide and other metal oxides containingminor proportions of alkalis and carbon. The carbon content of the flyash represents that portion of the carbon of the original total that didnot burn during the combustion in the furnace due to the short time ofexposure of the coal to combustion temperatures and to inconsistenciesin operation. Hence, depending on these and other factors, the fly ashmay contain from as little as less than 1% to as high as even 40% byweigh of carbon in isolated instances. A typical fly ash analysis is asfollows:

Silica, SiO₂ 54.6 Aluminum Oxide, Al2O₃ 23.3 Iron Oxide, Fe2O₃ 2.1Titanium Oxide, TiO₂ 1.4 Calcium Oxide, CaO 0.4 Magnesium Oxide, MgO 0.5Sodium Oxide, NA2O 0.2 Potassium Oxide, K2O 1.8 Sulfur Trioxide, SO₃ 0.3Phosphorous Pentoxide, P2O₅ <0.1 Barium Oxide, BaO <0.1 Magnesium Oxide,Mn2O₃ <0.1 Strontium Oxide, SrO <0.1 Total Carbon, C 12.4 Net IgnitionLoss (+)/Gain (−) +4.4 Total 101.4

Additionally, this mixture contains an available alkali equivalent Na2Oof 0.33, and an Iron Oxide of Fe3O₄ of 2.0. The low density ash of thepresent invention includes a major amount of frothy, solid particleswith both internal and external porosity or interstitial cavities suchas those shown on attached FIG. 2, and a minor amount of thick wallhollow particles as shown in FIG. 3 having both internal and externalpores. Still further, a minor amount of cenospheres, such as those shownin FIG. 4 are also collected in the low density ash of the presentinvention. More particularly, the present invention includes from 10% to70% of the total mixture by weight of frothy, solid particles such asthose shown in FIG. 2, from 10% to 70% of thick walled hollow particlessuch as those shown in FIG. 3, and less than 1% of cenospheres such asthose shown in FIG. 4. More particularly, and in one embodiment of theinvention, the mixture of low density fly ash includes from 20% to 60%of frothy, solid particles and from 10% to 50% of thick walled hollowparticles such as those shown in FIG. 3.

It has been found that the low density ash particles from the firsthopper are of a significantly lower density than the other ash particleswhich tend to fall out in hoppers 30B, 30C, etc. The accumulation ofmaterial in the first hopper 30A is of a good quality and is of a highpercentage, if not all, low density ash. The density of thisaccumulation in hopper 30A is at least 10% lower in density than raw flyash normally produced by electrostatic precipitation, and it has beenfound to be 35% or more lower in density.

In experiments, the accumulation of material had a specific gravity ofless than 2 and often is in the range of 1.6 to 1.99, which is a verygood grade low density filler material. The vast majority of particlescollected in hopper 30A were also about between 30 microns to 400microns in size. It is also noted that the cost of collection of suchlow density ash from the first hoppers 30A is very low, far less thanthe costs associated with other processes to separate ash by particlesize or density. Chemically, the composition of the low density ashparticles is similar to the composition of the overall ash populationpassing in the gas stream, and is lower in density as a result of itsfrothy form and structure which has both internal and external porosity,which ensures that there is a relatively low density associated with theproduct.

In an alternative embodiment, the first and second fields 14A and 14B,or first three fields 14A-14C may be de-energized. In this case, lowdensity fly ash is deposited in all three respective hoppers although agradual increase in the density generally occurs in each successivefield. Still further, the present invention may be operated by ensuringthat expansion nozzle 11 has a cross sectional configuration largeenough to decelerate the gas stream efficiently to ensure that lowerdensity particles will fall out of the gas stream and into hopper 30A.In this manner, de-energizing the electrostatic precipitator is notnecessary as the large diameter expansion nozzle 11 is sufficient toslow the gas stream and allow low density air entrained particles tofall out as a result of the drastic reduction in momentum. Expansionnozzle 11 may not be positioned adjacent an electrostatic precipitator,and simply decelerate the gas stream to allow low density particles tofall out of the stream. However, if expansion nozzle 11 is positionedadjacent a precipitator, then the collector plates must be de-energizedto allow low density particles to fall.

Accordingly, the improved low density ash separation and collectionmethod and device achieves all the enumerated objectives, does so withgreat cost efficiency and generates an entirely new low density fly ashmaterial with unique properties and characteristics as a result of thisnew art.

In the foregoing description, certain terms have been used for brevity,clearness and understanding; but no unnecessary limitations are to beimplied therefrom beyond the requirement of the prior art, because suchterms are used for descriptive purposes and are intended to be broadlyconstrued.

Moreover, the description and illustration of the invention is by way ofexample, and the scope of the invention is not limited to the exactdetails shown or described.

Having now described the features, discoveries and principles of theinvention, the manner in which the improved low density ash separationand collection method and device is constructed and used, thecharacteristics of the construction, and the advantageous, new anduseful results obtained; the new and useful structures, devices,elements, arrangements, parts and combinations, are set forth in theappended Claims.

What is claimed is:
 1. A method of separating a low density fly ashfraction from an overall group of fly ash entrained in a gas streamcomprising the steps of: passing the gas through a duct having a firstarea; passing the gas stream from the duct through an expansion nozzle;passing the gas stream through the expansion nozzle and into a hollowbody having a second area, whereby the second area is larger than thefirst area; decelerating the gas stream as it enters the expansionnozzle to separate out low density ash particles; collecting the lowdensity ash particles from a hopper positioned adjacent the expansionnozzle; wherein the expansion nozzle is positioned adjacent anelectrostatic precipitator; the electrostatic precipitator collects ashfrom the overall gas stream having a first ash density, and in which thelow density ash collected in the hopper positioned adjacent theexpansion nozzle is at least 10 percent lower in density than the firstash density produced by the electrostatic precipitator; the low densityash that is collected has a specific gravity of 2.0 or less; the lowdensity ash that is collected in the hopper positioned adjacent theexpansion nozzle has a particle size distribution that is predominantlyin the range of from between 30 microns to 400 microns; and the lowdensity ash that is collected includes a major amount of frothy,semi-solid particles with both internal and external porosity and aminor amount of hollow particles with thick walls containing internaland external pores.
 2. A method as defined in claim 1 in which thesecond area is in the range of from six to twenty-five times larger thanthe area of the duct.
 3. A method as defined in claim 2 in which thesecond area is in the range of from nine to fifteen times larger thanthe first area.
 4. The method as defined in claim 1, comprising thefurther steps of positioning the duct adjacent an electrostaticprecipitator having at least one electrostatic field; de-energizing atleast one electrostatic field in the electrostatic precipitator; andallowing the low density ash to fall into the hopper positioned adjacentthe expansion nozzle.
 5. The method as defined in claim 4 in which thede-energized field is the first field positioned adjacent the expansionnozzle in the electrostatic precipitator.
 6. The method as defined inclaim 4 in which at least the first two fields are de-energized in theelectrostatic precipitator.
 7. The method as defined in claim 1 in whichthe electrostatic precipitator has a maximum capacity for apre-determined gas stream level, and in which the electrostaticprecipitator is oversized in capacity with respect to saidpre-determined gas stream level flowing thereby.
 8. The method asdefined in claim 1 in which the electrostatic precipitator collects ashfrom the overall gas stream having a first ash density, and in which thelow density ash collected in the hopper positioned adjacent theexpansion nozzle is at least 35 percent lower in density than the firstash density produced by the electrostatic precipitator.
 9. The method asdefined in claim 1 in which the low density ash collected in the hopperpositioned adjacent the expansion nozzle has a specific gravity in therange of from 1.6 to 1.99.
 10. The method as defined in claim 1 in whichthe low density ash further includes a minor amount of cenospheres.